A SAP calculation is required where a material change of use is proposed.
Part L1b of the Building Regulation states that a material change of use is when:

The building is to be used as a dwelling when previously it was not.

The building contains a flat when previously it did not.

The building, which contains at least one dwelling, will contain a greater or lesser number of dwellings than it did previously

Calculations for new conversions can only be carried out by accredited Design SAP assessors, only full SAP calculations are accepted under the Building Regulations and an On-Construction EPC is required upon completion of the work

The DER/TER calculation in not required for a conversion. Compliance is based on controlled services/fittings and thermal elements.